1. Field of the Invention
The invention relates to minimizing resistance to catheter advancement during vascular cannulation.
2. Description of the Related Art
The high incidence of obtaining flashback via the needle but failing to thread the overlying catheter (“flashback yes/threading no”) is largely responsible for the reported incidence of 2.6 attempts for each successful intravenous (IV) catheter insertion. Failure to thread is attributable, in large part, to the difficulties encountered threading the catheter through a vessel wall.
FIG. 1 shows the features of currently available (prior art) needle/catheter combinations that are predispose to such difficulty: a) the catheter may still be outside the vessel wall at the time the needle opening is within the vessel and flashback has been obtained; b) the path in the anterior vessel wall through which the catheter must be thread is much smaller than the circumference of the catheter if only the needle tip and/or distal taper have passed through the vessel wall (since they are less wide than the shaft and overlying catheter). As those skilled in the art will appreciate, the “flashback-to-catheter” distance between the open needle bevel and the end of the catheter proximal to the needle bevel is critical; the longer this distance, the greater the likelihood that the catheter will not be in the vessel lumen (or within the anterior vessel wall) at the time of flashback.
At the time of flashback, the aforementioned scenario is most common. However, needle/vessel relationships at the time of flashback may range from:                1) At one extreme (cited above), only the narrow needle tip, with its open bevel, may be within the vessel. Threading of the overlying catheter through the anterior wall of the vessel would be prone to problems.        2) At the other extreme, the tip of the needle may have passed through the vessel lumen to the posterior wall of the vessel, while part of the bevel remains within the vessel. Threading would be impeded by the posterior vessel wall.        
It often is not clear which we way to go when the catheter initially fails to thread; i.e., should one advance the needle so as to increase vascular entry (to correct #1 above); should one withdraw the assembly from the distal wall (to correct #2 above)? A mistake is not only time-consuming; it can lead to catheter damage (burring, bending) and possible vessel trauma and, clearly, failed insertion.
Between the two extremes, the open needle bevel and tip may lie within the vessel lumen while the proximal bevel and/or all or part of the needle taper and/or the distal needle shaft (and overlying catheter) may undesirably be outside or within the vessel wall (as opposed to being within the vessel lumen). Hence the flashback-to-catheter distance is critical.
Thus, failure to thread an intravascular catheter despite attainment of flashback via the underlying needle is often the consequence of “a matter of millimeters:”                is the catheter appropriately in the vessel?        are the needle and catheter a few mm too far into the vessel such that catheter threading is inhibited by abutment against the distal vessel?        is the catheter a few mm short of the vessel even though the needle is intravascular?        is the path created by the needle in the anterior wall of the vessel too narrow?        
The answer typically is obtained by trial and error.
As noted above, in the aforementioned needle/vessel relationships, there not only is a difference in catheter location at the time of flashback, but also the size of the needle portion that has passed through the anterior vessel wall and hence size of the path along which the catheter must thread in the anterior wall of the vessel:                1) If only the narrow needle tip has reached the vessel, the path within the vessel wall likely would be relatively narrow since only a relatively narrow portion of the needle's distal taper would have passed through the anterior wall and thereby created a channel within said wall.        2) If the tapered region proximal to the tip reached the vessel, the path width would correspond to the changing width of the taper and thus range in caliber between the width of the taper and the width of the shaft.        3) If the shaft is fully within or through the vessel wall, then the path will conform to the circumference of the needle shaft (and allow for easiest threading through the anterior wall).        
The impact of these factors (e.g., the encumbrance of difficult catheter insertion and potentially bending the hanging end of the catheter as it advanced through the narrow path created by the needle taper and point) can best be appreciated by picturing what is occurring at the level of the skin (where it is readily visible). As it passes through the skin, the catheter remains adjacent to the needle shaft of the needle and there is no freely hanging catheter. However, if the a catheter is thread over a guidewire of significantly smaller circumference, then the catheter will get “hung up” at the skin unless a slit is made in the skin (with a knife—not practical for a vessel “hidden” under the skin and prone to injury and bleeding).
Catheter/Vessel Relationships
As discussed above in relation to FIG. 1, and as shown in U.S. Provisional Application Ser. No. 61/421,135, which is incorporated herein by reference, since the catheter that overlies the needle traditionally ends proximal to the proximal end of the needle's open bevel and taper, the catheter typically is not engaged with the vessel at the time that flashback via the needle initially is detected. Hence, the catheter will need to be advanced over the needle to enter the vessel. Catheter advancement over the shaft should be relatively simple for a catheter adjacent to a shaft which has created a channel in the vessel wall (that obviously is the circumference of the shaft). However, advancing the catheter through the vessel wall is more complicated when the catheter must be advanced if the needle taper and tip (as opposed to shaft) are in the anterior vessel wall:                this not only would result in a freely hanging catheter end as the catheter is advanced beyond the needle shaft (due to the gap between the circumference of the needle tip and taper vs. the catheter);        but also requires catheter advancement through a path whose circumference is much smaller than that of the catheter.        
Both factors predispose to impedance to catheter advancement through the vessel wall and the potential for bending or otherwise damaging the catheter tip.
As noted above, the problem, and hence the solution, lies within a ˜1-5 mm span on the needle/catheter combination; specifically, the flashback-to-catheter distance shown in FIG. 1. This distance for a given needle/catheter combination is relatively constant for a given needle gauge in that current combinations have a catheter which ends uniformly (i.e., without a slant conforming to the needle bevel) slightly proximal to a traditional bevel. It is easy to appreciate why modification of this configuration is critical in light of the aforementioned statements as to the importance of the location of the catheter at the time of flashback and the size of the path created by the needle in the vessel wall.
Attempts to Overcome the Problem—Needles with Recessed Orifice(s)
Attempts to overcome the problem posed by the flashback-to-catheter gap have focused primarily on the use of recessed orifices (e.g., a closed-tip pencil point needle with a recessed orifice on the needle and overlying catheter). However, this comes at potential “cost”:                unless specifically modified (as described in U.S. Provisional Application Ser. No. 61/421,135), the point of a pencil-point needle has the sharpness of a pencil (as opposed to a hypodermic needle);        it requires a change of insertion technique (since pencil points require a different angle of insertion);        it may require a greater length of insertion (to reach the recessed orifice), which may compromise catheter threading as there is increased likelihood that threading will be impeded by the posterior vessel wall (one inventor (Wiley et al.) proposed curving the back wall of the needle to minimize penetration of the distal wall of the vessel (it is believed this would further compromise the ability to pierce the skin and penetrate the vessel)); and        if the catheter is lengthened so as to reach an orifice on the needle shaft, then it likely would either be narrowed at its distal end, markedly slanted and/or freely hanging over the needle region of lesser circumference.        
Clearly, if a needle cannot enter the vessel, then ease of catheter threading becomes moot.
In U.S. Pat. No. 5,478,328 (in 1996), the inventors of the present application appreciated the potential for using a recessed-orifice pencil-point needle for catheter insertion. However, although it was mentioned that a catheter could be threaded, the main purpose of the catheter was to cover the orifices to minimize contamination. Additionally, a problem in the present context, the relative bluntness of a pencil-point needle was advantageous for needles designed to decrease the likelihood of causing healthcare worker injury. Specifically, in U.S. Pat. No. 5,478,328, the inventors of the present application recommended pencil point needles for healthcare worker safety in part because they are less sharp than standard hypodermic needles.
In U.S. Pat. No. 6,391,014, Silverman showed that puncturing the skin with a pencil-point needle requires more pressure than with a standard open-bevel hypodermic needle. During testing as shown in FIGS. 31 and 32 of the '014 patent, Silverman found that “a standard open-bevel needle consistently punctured the skin such that blood appeared (and pain was felt) at 9-10 mm Hg pressure. (Lesser forces, due to lower currents, did not cause skin puncture.) In contrast, a sharp closed tip (pencil-point) needle required a pressure of >15 mmHg to puncture the skin (and this was to a depth which did not advance the orifice into the skin).”
FIGS. 31 and 32 from Silverman (U.S. Pat. No. 6,391,014) shows means I've provided for testing needle sharpness. FIGS. 31a and 31b illustrate a device for determining the force required for a given needle to penetrate the skin (FIG. 31a) or a diaphragm (FIG. 31b) wherein the needle is mounted on a scale which records the force required for penetration. FIGS. 32a and 32b illustrate a more elaborate means of testing the penetrability of skin and diaphragms. The illustrated mechanism is designed to provide involuntary movements of the thumb by contraction of the adductor pollicis muscle as a result of stimulation of the ulnar nerve (analogous to the means used to assess neuromuscular weakness in patients undergoing general anesthesia). The force of contraction is recorded by a transducer (specifically an adductor pollicis force transducer). In the pictured embodiments, the needle is maintained in a fixed position in front of the thumb. In alternative embodiments, the needle can be secured to a thumb while the object to be punctured is maintained in a fixed position in front of it. During testing with the setup illustrated in FIG. 32, we found that a standard open-bevel needle consistently punctured the skin such that blood appeared (and pain was felt) at 9-10 mm Hg pressure. (Lesser forces, due to lower currents, did not cause skin puncture.) In contrast, a sharp closed tip needle required a pressure of >15 mmHg to puncture the skin (and this was to a depth which did not advance the orifice into the skin).
In addition to the recessed-orifice, pencil-point needle, another configuration for a recessed orifice needle had been proposed. In 1997, Tretola introduced an embodiment that provided a solid, beveled needle tip joined to a straight cylindrical body with a side hole proximally spaced from, but near the tip. The hole is carried on the needle body and the adjacent overlying catheter wall, near its distal end. This hole enables flashback to occur after the catheter has fully penetrated the vessel. However, this has not gained widespread acceptance and has been criticized by Wiley et al. (filed patent application, 2010) because                “the use of a solid tip dictates that the needle be inserted a significant distance into the vessel before any flashback occurs. This degree of insertion may, in fact, be excessive. In addition, the tip is constructed with the traditional bevel, having a point residing in line with the sidewall. In other words, the tip and point are simply extensions of the bottom side of the cylindrical needle shaft, and thus the structure of the needle still invites over-insertion as set forth in FIG. 2 [of Wiley] above.” The risk of over-insertion is, in fact, enhanced by the relatively proximal placement of the side hole. Moreover, the traditional beveled tip does nothing to reduce the damage potential from an over-penetrating insertion.”        